Ultrashallow photodiode using indium

ABSTRACT

The invention provides an imager having a p-n-p photodiode with an ultrashallow junction depth. A p+ junction layer of the photodiode is doped with indium to decrease transient enhanced diffusion effects, minimize fixed pattern noise and fill factor loss.

FIELD OF THE INVENTION

The present invention relates generally to image sensors and methods offabrication thereof and in particular to a p-n-p photodiode having anultrashallow p+ region doped with indium.

BACKGROUND

Typically, a digital imager circuit includes a focal plane array ofpixel cells, each one of the cells including a photoconversion device,e.g. a photogate, a photoconductor, or a photodiode. A CMOS imager isone such digital imager circuit and includes a readout circuit connectedto each pixel cell in the form of an output transistor. Thephotoconversion device converts photons to electrons which are typicallytransferred to a floating diffusion region, connected to the gate of asource follower output transistor. A charge transfer device can beincluded as well and may be a transistor for transferring charge fromthe photoconversion device to the floating diffusion region. Imagercells also typically have a transistor for resetting the floatingdiffusion region to a predetermined charge level prior to chargetransference. The output of the source follower transistor is gated asan output signal by a row select transistor.

Exemplary CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. No. 6,140,630 toRhodes, U.S. Pat. No. 6,376,868 to Rhodes, U.S. Pat. No. 6,310,366 toRhodes et al., U.S. Pat. No. 6,326,652 to Rhodes, U.S. Pat. No.6,204,524 to Rhodes, and U.S. Pat. No. 6,333,205 to Rhodes. Thedisclosures of each of the forgoing are hereby incorporated by referenceherein in their entirety.

FIG. 1 illustrates a block diagram of a CMOS imager device 308 having apixel array 200 with each pixel cell being constructed as describedabove. Pixel array 200 comprises a plurality of pixels arranged in apredetermined number of columns and rows (not shown). The pixels of eachrow in array 200 are all turned on at the same time by a row selectline, and the pixels of each column are selectively output by respectivecolumn select lines. A plurality of row and column lines are providedfor the entire array 200. The row lines are selectively activated by arow driver 210 in response to row address decoder 220. The column selectlines are selectively activated by a column driver 260 in response tocolumn address decoder 270. Thus, a row and column address is providedfor each pixel. The CMOS imager is operated by the timing and controlcircuit 250, which controls address decoders 220, 270 for selecting theappropriate row and column lines for pixel readout. The control circuit250 also controls the row and column driver circuitry 210, 260 such thatthese apply driving voltages to the drive transistors of the selectedrow and column lines. The pixel column signals, which typically includea pixel reset signal (V_(rst)) and a pixel image signal (V_(sig)), areread by a sample and hold circuit 261 associated with the column device260. A differential signal (V_(rst)−V_(sig)) is produced by differentialamplifier 262 for each pixel which is amplified and digitized by analogto digital converter 275 (ADC). The analog to digital converter 275supplies the digitized pixel signals to an image processor 280 whichforms a digital image.

In a CMOS imager, when incident light strikes the surface of aphotodiode, electron/hole pairs are generated in the p-n junction of thephotodiode. The generated electrons are collected in the n-type regionof the photodiode. Photo charge may be amplified when it moves from theinitial charge accumulation region to the floating diffusion region orit may be transferred to the floating diffusion region via a transfertransistor. The charge at the floating diffusion region is typicallyconverted to a pixel output voltage by the source follower transistordescribed above.

A conventional CMOS image sensor cell having a p-n-p photodiode is shownin FIG. 2. P-n-p photodiodes are an example of a type of photoconversiondevice typically used in CMOS image sensors. A p+ region 21 is shownabove an n-type region 23 to form the photodiode 49. Typically, the p+region 21 is implanted to create a p-n junction. A transfer transistorwith associated gate 26 and a reset transistor with associated gate 28are also shown, along with a floating diffusion region 16 andsource/drain region 30. A source follower transistor and row selecttransistor are also included in the 4-transistor (4-T) cell of FIG. 2,but are not shown in the cross-section depicted.

Imagers having p-n-p photodiodes may suffer from problems such asinefficient charge transfer and image lag due to potential barriersbetween the. photodiode 49 and transfer gate 26 region. Fill factor lossis also a problem associated with CMOS image sensors. Fill factor is ameasure of the ratio of electrons produced per given light intensity.Fill factor loss may occur when higher concentrations of p-type dopantsare used in the surface of a p-n-p photodiode and diffuse into n-typedopants, thereby compensating them and causing a reduction in fillfactor.

Another problem associated with CMOS image sensors is fixed patternnoise which causes static in the image due to pixel to pixel variations.Fixed pattern noise is created by a mismatch between transistorparametrics and photodiode characteristics between adjacent pixels.Variation in pinned voltage (V_(PIN)) and variation in barrier height inthe photodiode/transfer gate region also cause an increase in fixedpattern noise.

In order to address the problems discussed above, some CMOS imagesensors employ an ultra-shallow p-n junction where the p+ region of ap-n-p photodiode is implanted at a very shallow depth, close to thesurface of the substrate. Some advantages of an ultrashallow p-njunction are the ability to decrease the size of the pixel and improveresponsivity for colors, such as blue, with a shorter wavelength.Shallow p-n junctions may also minimize fixed pattern noise problems bykeeping the V_(PIN) variations from pixel to pixel to a minimum.

Ultrashallow p-n junctions have typically been formed using boron as thedopant in the p+ region. However, damage during ion implantation cancause boron dopants to diffuse by transient enhanced diffusion (TED). Asa consequence of TED, silicon interstitials are created when siliconatoms are dislodged from the crystal lattice and the implanted borondopants diffuse into the substrate farther than the intended implantarea. Transient enhanced diffusion is a problem associated withformulating ultrashallow junctions using boron as the p+ dopant.

SUMMARY

The invention provides a p-n-p photodiode for an imager cell with anultrashallow junction depth. The p+ junction layer is doped with a lowconcentration of indium to decrease transient enhanced diffusioneffects, and minimize fixed pattern noise and fill factor loss.

Additional features of the present invention will be apparent from thefollowing detailed description and drawings which illustrate exemplaryembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional CMOS imager chip having apixel array;

FIG. 2 is a cross-sectional view of a conventional pixel sensor cellhaving a p-n-p photodiode;

FIG. 3 is a cross-sectional view of an embodiment according to theinvention;

FIG. 4–FIG. 6 show cross-sectional views of a portion of a semiconductorwafer during various stages of processing in accordance with theinvention;

FIG. 7 is a schematic diagram of a processing system employing a CMOSimager having photodiodes constructed in accordance with the presentinvention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and show by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized, and thatstructural, logical, and electrical changes may be made withoutdeparting from the spirit and scope of the present invention. Theprogression of processing steps described is exemplary of embodiments ofthe invention; however, the sequence of steps is not limited to that setforth herein and may be changed as is known in the art, with theexception of steps necessarily occurring in a certain order.

The terms “wafer” and “substrate,” as used herein, are to be understoodas including silicon, silicon-on-insulator (SOI) or silicon-on-sapphire(SOS) technology, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor foundation, and othersemiconductor structures. Furthermore, when reference is made to a“wafer” or “substrate” in the following description, previous processingsteps may have been utilized to form regions, junctions, or materiallayers in or over the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, gallium arsenide or othersemiconductors.

The term “pixel,” as used herein, refers to a photo-element unit cellcontaining a photoconversion device for converting photons to anelectrical signal. For purposes of illustration, a single representativepixel and its manner of formation is illustrated in the figures anddescription herein; however, typically fabrication of a plurality oflike pixels proceeds simultaneously. Accordingly, the following detaileddescription is not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims.

In the following description, the invention is described in relation toa CMOS imager for convenience; however, the invention has widerapplicability to any photodiode of any imager cell. Now referring to thefigures, where like reference numbers designate like elements, FIG. 3illustrates a pixel sensor cell constructed in accordance with anembodiment of the invention. A photoconversion device 50 is formed in asubstrate 60 having a doped layer or well 61, which for exemplarypurposes is a p-type well. The photoconversion device is a photodiodeand may be a p-n junction photodiode, a Schottky photodiode, or anyother suitable photodiode, but for exemplary purposes is discussed as ap-n-p photodiode. In addition and for exemplary purposes only, substrate60 is a p-type substrate and well 61 is a p-type well more heavily dopedthan p-type substrate 60.

The exemplary pinned photodiode 50, as shown in FIG. 3, has a p+ region22 and an n-type region 24. Regions 22 and 24 meet to form a p-njunction at a distance d from the surface of substrate 60. The depth ofp+ region 22 is therefore measured form its starting point at thesurface of substrate 60 to the p-n junction, and is equal to d. In theexemplary embodiment of FIG. 3, p+ region 22 is implanted at anultrashallow depth defined as being a region having a depth from asurface of the substrate in the range of about 300 Å to 800 Å, andpreferably a depth of about 800 Å. A junction at such a depth issimilarly referred to herein as an ultrashallow junction. According toan exemplary embodiment of the invention, the p+ region 22 is implantedwith indium atoms in a concentration range of about 1×10¹⁸/cm³ to5×10¹⁸/cm³.

Indium has a heavier mass than other dopants such as boron and thusdiffuses less during backend thermal steps of formation. Indium, at thisconcentration range, therefore has better transient enhanced diffusioncharacteristics compared to boron. Indium typically acts as an acceptor(p-type) dopant when it occupies a substitutional site in the crystallattice. However, when indium is in an interstitial site, it acts as adonor (n-type). When implanted into an ultrashallow junction, indiumthat diffuses into interstitial regions will then act as a donor,thereby reducing fill factor loss and minimizing TED effects. The use ofindium also reduces fixed pattern noise by minimizing variations inbarrier height and V_(PIN).

The remaining structures shown in FIG. 3 include a transfer transistorwith associated gate 26 and a reset transistor with associated gate 28.Source/drain regions 30, floating diffusion region 16 and shallow trenchisolation (STI) regions 55 are also shown. A source follower transistor27 and row select transistor 29 with associated gates are also includedin the pixel sensor cell but are not shown in the FIG. 3 cross-sectionalview. They are instead depicted in FIG. 3 in electrical schematic formwith the output of the row select transistor 29 being connected with acolumn line 31. Although shown in FIG. 3 as a 4-transistor (4T)configuration with a transfer transistor, the invention can also beutilized in a 3-transistor (3T) configuration, without a transfertransistor, or in other higher number transistor configurations.

FIGS. 4–6 show one exemplary method of forming a CMOS image sensor withan ultra-shallow p-n-p photodiode in accordance with the presentinvention at various stages of processing. For convenience, the samecross-sectional view of FIG. 3 is utilized in FIGS. 4–6 for the ensuingdescription, so the source follower and row select transistors are notillustrated. The invention will be described as formed in a p-well of ap-type substrate; however the invention may also be formed in an n-wellin an n-type substrate, and other photosensor structures may also beused.

First a substrate 60, as shown in FIG. 4, is formed. This substrate 60is a p-type silicon substrate as shown in FIG. 4 with a p-well 61 formedtherein. The p-type well 61 may be formed before or after the formationof isolation regions 55 in substrate 60. The p-well implant may beconducted so that the pixel array well 61 and a p-type periphery logicwell, which will contain logic circuits for controlling the pixel array,have different doping profiles. As known in the art, multiple highenergy implants may be used to tailor the profile of the p-type well 61.The p-well 61 is doped to a higher dopant concentration than the p-typesubstrate 60.

Isolation regions 55, also shown in FIG. 4, are formed to electricallyisolate regions of the substrate where pixel cells will later be formed.The isolation regions 55 can be formed by any known technique such asthermal oxidation of the underlying silicon in a LOCOS process, or byetching trenches and filling them with oxide in an STI (shallow trenchisolation) process. Following formation of isolation regions 55 if thep-type well 61 has not yet been formed, it may then be formed by blanketimplantation or by masked implantation to produce the p-type well 61.

Next the circuitry of the pixel sensor cell, including the transfertransistor gates; reset transistor gates; the source follower transistorgates; and the row select transistor gates are formed by well-knownmethods, e.g., blanket deposition of gate oxide, doped polysilicon,deposition of metal for a silicide, annealing to form a silicide, thenpatterning and etching. FIG. 5 shows an exemplary embodiment with formedgate stacks 15, 19 for transfer transistor 26 and reset transistor 28respectively. Although shown as a 4-transistor (4T) embodiment, theinvention can also be used in a 3-transistor (3T) embodiment or otherpixels having five or more transistors.

Floating diffusion region 16 and source/drain regions 30 are also formedafter the gate stacks 15, 19. Doped regions 30 and 16, shown in FIG. 5,are formed in the p-well 61 and are doped to an n-type conductivity. Forexemplary purposes, doped regions 30, 16, are n+ doped and may be formedby applying a mask to the substrate and doping the regions 30, 16 by ionimplantation.

After source/drain regions 30 and floating diffusion region 16 implantsare formed, the gate stack sidewall insulators are then formed on thesides of the gate stacks using known techniques. FIG. 6 shows theinsulating sidewalls added to the gate stacks for transfer transistor 26and reset transistor 28. FIG. 6 also shows implantation of thephotodiode 50 by methods known in the art. Region 24 is implanted withn-type dopants while a p-type dopant, indium, is implanted to formregion 22 an ultrashallow depth, defined as being a region having adepth from a surface of the substrate to a p-n junction in the range ofabout 300 Å to 800 Å, and preferably a depth of about 800 Å. The dopingconcentration in region 22 for indium is around 1×10¹⁸/cm³ to5×10¹⁸/cm³. A spike anneal with a fast ramp up to a high temperature,for example, 250° C. per second, may be performed after the ionimplantation in order to improve the transient enhanced diffusioncharacteristics of the indium implant.

FIG. 6 also illustrates schematically a number of indium ions inphotodiode 50 in the silicon crystal lattice of substrate 60. Indium ion70 is illustratively one of a number of ions in region 22, while indiumion 72 is illustratively one of a number of ions in region 24. Indiumion 70 and other indium ions in region 22 occupy substitutional sites inthe lattice and therefore act as acceptors of electrons. Indium ion 72and other indium ions that diffuse into the n-type region 24 occupyinterstitial sites and act as donors. Since indium ions can act asacceptors in region 22 and donors in region 24, the fill factor of thepixel cell is thereby increased.

The pixel sensor cell is essentially complete at this stage, andconventional processing methods may be used to form insulating,shielding, and metallization layers to connect gate lines and otherconnections to the pixel sensor cells. For example, the entire surfacemay be covered with a passivation layer 88 of, for example, silicondioxide, BSG, PSG, or BPSG, which is CMP planarized and etched toprovide contact holes, which are then metallized to provide contacts.Conventional layers of conductors and insulators may also be used tointerconnect the structures and to connect the pixel to peripheralcircuitry.

FIG. 7 shows a processor system 300, which includes an imager device 308having the overall structure of FIG. 1, but having an array of pixelsconstructed in accordance with the invention. The imager device 308 mayreceive control or other data from system 300. System 300 includes aprocessor 302 having a central processing unit (CPU) that communicateswith various devices over a bus 304. Some of the devices connected tothe bus 304 provide communication into and out of the system 300; aninput/output (I/O) device 306 and imager device 308 are suchcommunication devices. Other devices connected to the bus 304 providememory, illustratively including a random access memory (RAM) 310, harddrive 312, and one or more peripheral memory devices such as a floppydisk drive 314 and compact disk (CD) drive 316. The imager device 308may be constructed as shown in FIG. 1 with the pixel array 200 havingthe characteristics of the invention as described above in connectionwith FIGS. 3–6. The imager device 308 may, in turn, be coupled toprocessor 302 for image processing, or other image handling operations.

The processes and devices described above illustrate preferred methodsand typical devices of many that could be used and produced. The abovedescription and drawings illustrate embodiments, which achieve theobjects, features, and advantages of the present invention. However, itis not intended that the present invention be strictly limited to theabove-described and illustrated embodiments. Any modifications, thoughpresently unforeseeable, of the present invention that come within thespirit and scope of the following claims should be considered part ofthe present invention.

1. A photoconversion device comprising: a substrate having a surface; afirst doped region having a first conductivity type and located belowthe surface of the substrate at an ultrashallow depth, wherein saidfirst doped region is doped with indium; and a second doped regionhaving a second conductivity type located beneath said first dopedregion for producing photogenerated charges, wherein said first dopedregion is implanted to a depth of about 800 Å.
 2. A photoconversiondevice comprising: a substrate having a surface; a first doped regionhaving a first conductivity type and located below the surface of thesubstrate at an ultrashallow depth; a second doped region having asecond conductivity type located beneath said first doped region forproducing photogenerated charges; and the first and second doped regionsboth including indium ions, the indium ions in the first region actingas electron acceptors, the indium ions in the second region acting aselectron donors.
 3. The system according to claim 2 wherein said firstconductivity type is p-type.
 4. The photoconversion device of claim 2wherein said first doped region is implanted with indium to a depth ofabout 800 Å.
 5. The photoconversion device of claim 2 wherein theconcentration of said indium dopant is in the range of about 1×10¹⁸/cm³to 5×10¹⁸/cm³.
 6. A photo diode comprising: a substrate having asurface; a first doped region having a first conductivity type andlocated below the surface of the substrate at an ultrashallow depth; anda second doped region having a second conductivity type located beneathsaid first doped region; and the first and second doped regions bothincluding indium ions, the indium ions in the first region acting aselectron acceptors, the indium ions in the second region acting aselectron donors, wherein the second doped region collects photo diodegenerated charges.
 7. The diode of claim 6 wherein said firstconductivity type is p-type.
 8. The diode of claim 6 wherein said firstdoped region is implanted to a depth in the range of about 800 Å.
 9. Thediode of claim 6 wherein the concentration of said indium dopant is inthe range of about 1×10¹⁸/cm³ to 5×10¹⁸/cm³.
 10. A processing systemcomprising: a processor; and an imager coupled to said processor, saidimager comprising: a substrate having a surface; a photosensitive areawithin said substrate for accumulating photo generated charge in saidarea, said photosensitive area having a first doped region having afirst conductivity type and located below the surface of the substrateat an ultrashallow depth and a second doped region having a secondconductivity type located beneath said first doped region; and a readoutcircuit comprising at least an output transistor formed on saidsubstrate, wherein said first and second doped regions are doped withindium.
 11. The system according to claim 10 wherein said firstconductivity type is p-type.
 12. The system according to claim 10wherein said first doped region is implanted to a depth in the range ofabout 800 Å.
 13. The system according to claim 10 wherein theconcentration of said indium dopant is in the range of about 1×10¹⁸/cm³to 5×10¹⁸/cm³.
 14. An imager comprising: a substrate having a surface;an array of pixel sensor cells formed on said substrate; each pixelsensor cell comprising: a photosensitive area within said substrate foraccumulating photo generated charge in said area, said photosensitivearea having a first doped region having a first conductivity type andlocated below the surface of the substrate at an ultrashallow depth anda second doped region having a second conductivity type located beneathsaid first doped region; and signal processing circuitry formed in saidsubstrate and electrically connected to the array for receiving andprocessing signals representing an image output by pixels of the arrayand for providing output data representing said image, wherein saidfirst and second doped regions are doped with indium.
 15. The imageraccording to claim 14 wherein said first conductivity type is p-type.16. The imager according to claim 14 wherein said first doped region isimplanted to a depth in the range of about 800 Å.
 17. The imageraccording to claim 14 wherein the concentration of said indium dopant isin the range of about 1×10¹⁸/cm³ to 5×10¹⁸/cm³.